A magnetic bubble memory operative in the field access mode was first disclosed in U.S. Pat. No. 3,534,347, of A. H. Bobeck, issued Oct. 13, 1970. Commercial memories of this type are organized in what is called a "major-minor" configuration as first disclosed in U.S. Pat. No. 3,618,054, of P. I. Bonyhard, U. F. Gianola, and A. J. Perneski, issued Nov. 2, 1971.
A bubble memory organized in the major-minor configuration comprises closed loop paths for recirculating patterns of magnetic bubbles. The paths are defined by magnetically soft elements (permalloy) or by patterns of ion implanted regions which move bubbles in response to a magnetic field reorienting (usually rotating) in the plane of bubble movement. The closed loops are called "minor loops" and are operative to move bubbles through reference positions at which bubble transfers to or from the loops occur.
Typically the minor loops are organized in parallel to move a pattern of bubbles into the reference positions during a read operation for transfer to a read out path as is well known. The read out path is called the "major" loop or path and extends between a bubble generator and, typically, an expander-detector arrangement of the type disclosed, for example, in U.S. Pat. No. 3,810,132 of A. H. Bobeck, issued May 7, 1974. In some organizations, the major path is separated into two paths, one called the "write line" originating at a bubble generator. The other is called the "read line" and terminates at a detector.
Frequently, a swap function is defined to permit transfer of bubble patterns between the reference positions in the minor loops and associated stages in the major path. In organizations where the major path is separated into two paths, the associated stages are defined in the write line. Such a swap function is disclosed in U.S. Pat. No. 4,007,453, of P. I. Bonyhard, Y. S. Chen and J. L. Smith, issued Feb. 8, 1977. The swap function results, for example, in a newly written bubble pattern in the associated stages being swapped for a bubble pattern occupying the reference positions in the minor loops when the swap occurs. But a problem exists in that minor loops cannot be located conveniently close enough to permit adjacent ones of the reference positions to coincide with consecutive stages of the write line. Consequently, swapping of information requires the placement of data in alternate stages, rather than in adjacent stages, of the write line. Consequently, read operations from the associated read line can occur only on every other cycle of the rotating in-plane field. This results in a reduction of the data rate of the bubble memory.
Various arrangements have been devised for permitting read-on-every-cycle operation. One such arrangement organizes the pattern of magnetic elements and electrical conductors such that bubbles can be located in stages between swap functions when a swap operation occurs and still do not interfere with the swap operation. We have found that we obtain better operating margins with our present design in which bubbles are stored only in alternative stages.